The present invention relates to a pilot-operated quick exhaust valve for use in conjunction with a control device that is used to control the flow of pneumatic or hydraulic fluid to an actuator of the kind used to operate the position of a valve.
In many applications it is desirable to automate the actuation of a pipeline valve via a remote control system. This is particularly necessary in harsh environments such as, for example, a petrochemical pipeline located on land or off-shore. The operation of, for example, a ball valve in such a pipeline is often effected by a valve positioner that provides fluid (typically pneumatic) signals to an actuator for operating the valve. A compressor delivers compressed air via a filter regulator to the positioner which controls the onward flow to the actuator by reference to position feedback signals. The regulator is required to reduce the available pressure from the compressor to a safe working level for the downstream positioner and associated pneumatic circuit. Regulators generally have a built-in filter to remove contaminants such as oil, water vapour and particulate matter.
The actuator typically comprises a piston and cylinder arrangement with a shaft associated with the piston being connected to the valve being operated. The piston divides the cylinder into a pair of chambers at least one of which may be selectively pressurised by the introduction compressed air in order to move the piston and therefore the shaft. In a single-acting piston, the other chamber is occupied by a biasing member such as a spring against which the pressurised air acts. When the pressurised air supply drops below a certain value the force applied by it to one side of the piston is less than that applied on the other side by the spring in which case the pressurised air is exhausted from the cylinder. In a double-acting piston air is selectively supplied to one of the chambers and simultaneously exhausted from the other.
The positioner generates pneumatic control signals which are not, in some applications, of sufficient volumetric flow rate to operate the actuator in the desired time period. It is therefore often necessary to employ a volume booster to ensure there is a sufficient sustained volume of fluid available to the actuator to ensure a rapid response time. Volume boosters are generally controlled by a pneumatic pilot signal received from the positioner and ensure that the pressure and volumetric flow of the fluid delivered to the actuator is sustained to achieve the desired actuator stroke speed. Separate flow regulators are often connected to the booster and this serves to increase the complexity of the system in terms of installation, servicing, maintenance and operation.
The pilot signals are generated by the positioner in response to a command signal directing the positioner to move the valve to a desired position. The command signal may be an open-loop signal or a closed-loop feedback electrical control signal that takes into account the position of the actuator. In an alternative arrangement the booster may be controlled directly by an electrical signal that operates a solenoid valve in the booster.
A known form of volume booster comprises a housing having an operating air inlet and outlet, both of which are in communication with the flow of operating air to the actuator, and a pilot signal inlet connected to an output of the positioner. The communication between the operating air inlet and outlet is selectively interrupted by a supply valve whose position is controlled by diaphragm assembly on which the pilot signal acts. The supply valve is connected to one end of a reciprocal valve stein the other end of which serves to open or close an exhaust valve in an exhaust passage defined in the diaphragm assembly. The pilot signal acts on one side of the diaphragm assembly whereas the outlet air pressure acts on the other side of the diaphragm assembly by virtue of a bleed passage in the housing from the outlet. In the event that the force applied by the pilot signal pressure to the first side of the diaphragm assembly exceeds that applied on the other side by the outlet pressure, the force differential serves to move the diaphragm assembly and valve stem to a first position in which the supply valve is open and the exhaust valve remains closed. Operating air can then flow from inlet to outlet so as to drive the actuator and position the valve. When the outlet pressure increases or the pilot signal pressure decreases to the extent that the forces on the diaphragm assembly cause it to move in the opposite direction, the diaphragm assembly moves to a second position in which the supply valve is closed and the diaphragm assembly lifts off the exhaust valve so that excess pressure can vent between the diaphragm assembly and the exhaust valve to the exhaust passage in the diaphragm assembly. The exhaust valve may be defined by a simple poppet valve on the end of the valve stem that seals against a seat defined at a bore in the diaphragm assembly. The location of the exhaust valve and passage means they tend to be relatively small and thus serve to restrict flow. The flow rate is significantly lower than that of the main flow leading to a slow reaction time. This is particularly undesirable in the event of an emergency where it is necessary to vent large volumes of air.
One solution to the problem of restricted exhaust flow is to provide a separate exhaust flow having a capacity equivalent to the main operating air flow. This may be achieved by using another booster or a quick exhaust valve both of which involve additional components, space and expense.
In one example of a separate exhaust capacity, an external conduit disposed outside the main body of the booster housing interconnects the outlet and the exhaust passage which are provided on opposite sides of the diaphragm assembly. An example of this is pneumatic volume booster Model 200XLR available from Fairchild Industrial Products Company of Winston-Salem, N.C., USA). Without the restriction imposed by the space within the body of the booster the external conduit can have a relatively large size so as to permit the exhaust flow to be as large as the main flow. This solution is relatively large and cumbersome and can therefore be disadvantageous in applications where there are space constraints.
The components of the pneumatic circuit comprising at least the regulator, positioner, volume booster and any directional control valves, are typically supported by suitable brackets on a back plate that is housed in a convenient area of a control room. It is desirable for the spaced occupied by such a circuit to be reduced as far as is possible.
The pressurised air is generally supplied to the actuator to advance the piston within the cylinder via a pneumatic supply circuit, including one or more control valves. For the piston to move in the reverse direction air is exhausted from the cylinder either through the supply circuit or in some instances it is desirable to employ a separate exhaust line with a quick exhaust valve so that air can be exhausted rapidly to atmosphere. The latter option eliminates the need for exhaust air to be directed through the control valve(s) in the supply circuit in which case the flow rate is restricted. Quick exhaust in this manner is particularly desirable when there is an emergency that requires the actuator to close the pipeline valve as quickly as possible.
Quick exhaust valves typically comprise a diaphragm supported for movement in a valve body. When the control valve is operated in a supply mode, the inlet of the valve body is connected to the air supply and the diaphragm is forced into a sealing position against an exhaust port so that the supply of pressurised air flows from the inlet to an outlet and enters the cylinder of the actuator. When the control valve is operated in an exhaust mode, there is an absence of air pressure at the inlet and the cylinder air pressure at the outlet forces the diaphragm away from the exhaust port so that air from the cylinder can pass through the valve to atmosphere.
Quick exhaust valves of the kind described above do not generally provide any modulation of exhaust flow: the exhaust outlet is either fully open to allow quick exhaust of air in the cylinder or fully closed to permit pressurisation of the cylinder.
It is an object of the present invention to obviate or mitigate the above, and other, disadvantages. It is also an object of one aspect of the present invention to provide for an improved, or alternative, fluid flow control device. It is an object of another aspect of the present invention to provide for an improved, or alternative, exhaust valve.